In wireless communications systems, wireless terminals and base stations are designed to transmit and receive radio frequency (RF) signals that propagate through RF environments. Depending on the type of wireless communications system and the services offered by the wireless system, the wireless terminals and base stations are equipped to perform specific signal-processing functions. For example, some wireless systems are required to identify the geographical location (i.e. geolocation) of the wireless terminals communicating on the system. Such wireless systems have been referred to as geolocation systems. The term geolocation as used herein refers to the point in two or three-dimensional space defined by a set of coordinates, e.g. longitude and latitude, and/or defined by a vector, i.e. distance and direction, from a known point in space.
Some conventional geolocation systems identify the geolocation of a wireless terminal by determining the time-of-arrival of the line-of-sight component of a signal transmitted by the wireless terminal. The line-of-sight component of the transmitted signal is that component of the signal that propagated directly from the wireless terminal to the location at which the signal was received (e.g. base station) without scattering or reflecting off structures in the RF environment. The term scattering refers to the phenomenon wherein an RF signal, traveling in an RF environment, hits and reflects of structures in the RF environment, thereby causing the RF signal to take random paths through the RF environment. This so-called multipath phenomenon can cause the incoming signal to be composed of several repeated versions of the transmitted signal, each version being a multipath component of the incoming signal.
To determine the time-of-arrival of the line-of-sight component of the transmitted signal, such geolocation systems receive the transmitted signal and pass the so-called incoming signal through a matched filter. The matched filter generates a correlation value based on a comparison of the shape of the waveform of the incoming signal to the shape of the waveform of the transmitted signal. The correlation value essentially peaks each time the matched filter determines that the shape of the waveform of the incoming signal is similar to or matches the shape of the waveform of the transmitted signal. Each time the correlation value reaches a peak value, the geolocation system identifies that time as the time-of-arrival of a multipath component of the incoming signal. As a result, since the line-of-sight component of the transmitted signal travels directly to the location of the receiving unit, such conventional geolocation systems assume that the time-of-arrival of the line-of-sight component of the incoming signal is the time at which the correlation value reaches its first peak.
It is the time-of-arrival of the line-of-sight component of the incoming signal that the above-described conventional geolocation systems use to identify the geolocation of the wireless terminal. In particular, the geolocation systems identifies the time-of-arrival of the line-of-sight component of the incoming signal received at a plurality of locations, and processes the various times of arrival to determine the distance of the wireless terminal from each of, for example, three receiver locations. From this "distance" information, the geolocation system determines the geolocation of the wireless terminal itself
These geolocation systems, however, are hindered by their failure to consider the effects of scattering in determining the times-of-arrival of the line-of-sight components. As described above, scattering may cause a signal to multipath and thus cause the incoming signal to be composed of a plurality of multipath components that arrive at different times. Depending on the amount of time between the respective multipath components of the incoming signal, the matched filter of the conventional geolocation system may not be able to accurately identify the time-of-arrival of the line-of-sight component. That is, the matched filter may not be able to distinguish between the time of arrival of the line-of-sight component and the next-arriving component of the incoming signal. When this happens, the matched filter may mistakenly view the line-of-sight component and the next-arriving component as a single multipath component. As a result, the correlator value will reach a peak somewhere in between the actual time-of-arrival of the line-of-sight component and the time-of-arrival of the next-arriving multipath component. This will cause the geolocation system to mistakenly assume that the time-of-arrival of the line-of-sight component was received at a later, or "time-shifted," time than it actually arrived. Such a mistake could substantially reduce the accuracy of the processing performed by the geolocation system in calculating the geolocation of the wireless terminal.